1. Field of the Invention
This invention relates to the elimination or reduction of appearance defects known as "measles", as defined hereinafter, in fluorescent lamps having a conductive starting aid layer or coating on the inner surface of the lamp tube or glass envelope.
2. Background of the Invention
Rapid-start or similar fluorescent lamps including an internal conductive layer, such as a tin oxide or indium oxide layer, and mercury vapor as part of the discharge sustaining gas fill are subject to the formation of localized appearance defects referred to as "measles." Such defects comprise a dark spot surrounded by a concentric ring of discoloration usually on the order of one or two millimeters in diameter. Measles are believed to develop during lamp operation as a result of an interaction involving the conductive layer and the mercury in arc discharge. The mercury is presumed to penetrate the phosphor layer or coating leading to conditions which allow build-up of charge and subsequent discharge which results in the measle defect by disrupting the phosphor layer and generally forming a small crater in the glass tube.
The occurrence of such appearance defects has been delayed in fluorescent lamps having a tin oxide conductive layer by varying the electrical resistance of the conductive layer along the axial length of the glass tube. More particularly, the electrical resistance profile of the conductive layer has been varied from a flat or constant value to a U-shaped or "bathtub" profile wherein a relatively low resistance value is provided at the center portion of the lamp and relatively high resistance values are provided at the end portions of the lamp. The bathtub resistance profile is difficult to control and to uniformly maintain in a commercially acceptable manner using existing production equipment and technology. The relative differences in electrical resistance along the axial length of the lamps achieved in this manner tend to decrease after about the first 500 hours of lamp operation. Moreover, the resulting variations in electrical resistance merely delay the occurrence of such defects from a time following the first 1000 hours of lamp operation to a later time after about 3000 to 4000 hours of lamp operation. This is a rather short improvement in the total life of the lamp life which is in the order of about 20,000 hours. Accordingly, this process technique does not provide a satisfactory solution to such measle defects.
A variety of protective or barrier layers are known in the art for inhibiting or delaying other appearance defects characterized by darkened stains or a general discoloration of the phosphor layer and/or conductive layer. U.S. Pat. No. 3,624,444 discloses the use of a protective layer over a tin oxide conductive layer in a low pressure mercury vapor discharge lamp to inhibit black stains formed on the inner side of the glass tube. The protective layer is formed of oxides of elements of the secondary groups in columns 4 and 5 of the periodic table of elements, preferably titanium dioxide and zirconium dioxide. In U.S. Pat. No. 4,338,544, an aluminum oxide protective or barrier coating is taught to inhibit a "blackening" phenomenon on the tin oxide coating attributed to its reaction with mercury. U.S. Pat. No. 3,967,153 discloses a fluorescent lamp having an alumina layer deposited by application of a suspension of aluminum oxide over the tin or indium oxide conductive coating, with a layer of phosphor covering the alumina. U.S. Pat. No. 4,338,544 discloses a similar protective coating in a fluorescent lamp which further comprises an inert gas, such as krypton, neon or xenon, used together with mercury as the gas fill in the lamp. U.S. Pat. No. 4,363,998 likewise discloses the use of an alumina coating applied over the tin oxide coating, but also comprises the use of antimony oxide mixed with the alumina, the antimony oxide acting to improve the performance of a zinc silicate phosphor applied over the alumina-antimony oxide layer.
Thus it is known in the art to employ a layer of alumina, or certain other metal oxides, as a protective layer or precoat over the layer of conductive material to prevent its discoloration and/or that of the subsequently applied phosphor materials. However, such precoats of metal oxides have not effectively prevented or reduced the occurrence of measle defects.